Aquaculture is a rapidly growing global industry. The conventional fish catches are decreasing dramatically and the future requirement for seafood must be met through aquaculture.
However, in recent years, several challenges have arisen, with which this industry is faced. For salmonid-farming this concerns, among other things, the treatment of fish infected with crustacean parasites such as salmon louse, the escape of fish from floating farming cages, diseases due to viruses, bacteria and parasites, so-called “genetic pollution” of wild populations of salmonids, and emissions of waste products from the net cages to recipients. These challenges are particularly relevant to the farming of fish in conventional, floating flow-through cages. Such open net cages consist of a net forming a closed enclosure for fish in particular and being held afloat by means of an encircling buoyancy system.
Buoyancy systems for floating net cages may be divided into two main groups, so-called steel cages and so-called plastic cages. Steel cages consist of rectangular walkways in steel provided with floats on their bottom side. Each float may have the shape of a rectangular box. The walkways are joined together by hinges. The steel cages form a grid with longitudinal and transverse walkways. Each side of a square may be, for example, 10 m or 12 m long. The seines, which form closed enclosures, are placed in the grid and attached to the steel cages on hooks projecting from special posts or supports. Steel cages are also provided with railing, and the net may be attached to a handrail with hooks or lashings. The upper edge of the seine is thereby raised above the water surface, thus also forming a jump-fence to prevent fish from escaping from the net cage by jumping over the edge. The walkway forms a relatively stable platform for the movement of personnel and equipment. Equipment and feed can be stored on the walkways.
A plastic cage consists of at least one plastic pipe which is welded together into a ring. The plastic cage usually consists of two concentric rings. Plastic cages with three concentric plastic rings are known as well. The plastic pipe may be a continuous plastic pipe which is joined together at the ends into a ring. The plastic pipe may also be formed from straight pipe sections which are welded together into a polygonal ring. The ring may be octagonal, decagonal and so on.
The plastic rings are connected with radially oriented brackets in plastic or steel. Walkways may be placed on top of two concentric rings. The seine, forming a closed enclosure, is placed within the innermost pipe in the buoyancy system and fixed with projecting seine hooks. The seine hooks may be attached to the pipe or to a railing projecting up from the buoyancy system. The circumference of the seine in a plastic cage may be, for example, between 90 m and 160 m long, corresponding to a diameter of between 30 m and 50 m. The plastic rings are formed in fixed lengths and cannot very easily be length-adjusted. The walkway is relatively narrow. The rings and the walkway will follow the wave motions. The walkway is not suitable for storing equipment and only relatively light equipment can be moved along the walkway. The walkway extends around only one net cage. Personnel are dependent on a boat in order to move from one net cage over to another net cage.
Closed farming cages are known in the art. These are formed from a tight cloth material which forms a closed enclosure for aquatic organisms such as fish. To ensure that the water exchange rate is sufficient to maintain a minimum oxygen level in the water inside the cage, it is common to pump so much water in that the water surface inside the cage is higher than the water surface outside the cage. The pressure inside the cage is thereby greater than the ambient pressure and water will flow out of the cage through openings formed. This subjects the buoyancy system of a closed cage to greater forces than the buoyancy system of an open cage of the same size. In addition to keeping the cloth or net itself of the cage afloat, the buoyancy of the buoyancy system must be dimensioned for holding the amount of water inside the cage that is above the water surface of the surrounding water. This water constitutes a considerable mass. In addition, this water has a moment of inertia that causes the wave influence on the buoyancy system to be greater than in an open cage in which the wave motion substantially unobstructedly passes the buoyancy system and into the cage.
The nets of open net cages are attached to the buoyancy system by the buoyancy system being provided with projecting hooks. The buoyancy system may be provided with a handrail and the projecting hooks may be attached to the handrail. From so-called steel cages it is known to use special posts or supports provided with such hooks. The hooks may also be attached to floating rings which form a buoyancy system.
The patent publication GB 2068847 discloses a plurality of rectangular buoyancy elements made of concrete, which are held together by a wire or chain. The wire or chain extends through flush, elongated channels inside the buoyancy elements. In their adjacent end faces, the elements are formed with recesses at the mouths of the channels. A spacer formed of an elastic material is positioned in the recesses of the two adjacent end faces between two neighbouring buoyancy elements. The spacer is provided with a through bore for the wire or chain. The elastic spacer absorbs longitudinal forces so that neighbouring elements will not collide, counteracts relative vertical and horizontal lateral movements between two neighbouring elements, while, at the same time, allowing a certain degree of twisting and rotation between two neighbouring elements.